1. Field of the Invention
The present invention relates to filter devices including a balanced filter, and more specifically, relates to a filter device including a compensation circuit through which a signal path to a first balanced terminal and a signal path to a second balanced terminal are connected to each other.
2. Description of the Related Art
To date, in a duplexer of a cellular phone, a reception filter is formed of a balanced filter in many cases.
Japanese Unexamined Patent Application Publication No. 2011-160203 discloses a duplexer illustrated in FIG. 19. In a duplexer 1001, a transmission filter 1004 is connected between an antenna terminal 1002 and a transmission terminal 1003. A reception filter 1005 is connected to the antenna terminal 1002. The reception filter 1005 has a balanced-unbalanced transforming function. In other words, one end of the reception filter 1005 is connected to the antenna terminal 1002. The reception filter 1005 is connected to a first balanced terminal 1006 and a second balanced terminal 1007, which function as reception terminals.
As illustrated, the transmission filter 1004 has a ladder circuit configuration including a plurality of series arm resonators and parallel arm resonators. On the other hand, the reception filter 1005 is formed of a longitudinally coupled resonator-type balanced filter.
In the duplexer 1001, a compensation circuit 1008 is connected between the transmission terminal 1003 and the first and second balanced terminals 1006 and 1007. The compensation circuit 1008 includes a capacitor 1009 and a capacitor 1010. The capacitor 1009 and the capacitor 1010 are connected in series with each other between the first balanced terminal 1006 and the second balanced terminal 1007. A connection node between the first and second capacitors 1009 and 1010 is connected to the transmission terminal 1003.
It is stated in Japanese Unexamined Patent Application Publication No. 2011-160203 that, by using the compensation circuit 1008 described above, isolation at the time when the first balanced terminal 1006 functioning as a reception terminal is viewed from the transmission terminal 1003 and isolation at the time when the second balanced terminal 1007 is viewed from the transmission terminal 1003 are improved. Further, it is also stated that differential isolation obtained by synthesizing the isolations of the two paths using a balun is also improved.
It is stated in Japanese Unexamined Patent Application Publication No. 2011-160203 that differential mode isolation can be improved. However, the improvement effect of differential mode isolation in Japanese Unexamined Patent Application Publication No. 2011-160203 is not sufficient, and it is strongly required that differential mode isolation be further improved. In addition, it is difficult to improve differential mode isolation while suppressing degradation of insertion loss in a specific band.